Logistics of exterior roof insulation

Kamboji|发布General Questions上April 10, 2014 10:12am

Dear GBA readers,
I’m starting the process of finding a roofer who can add up to 6 inches of insulation on top of the roof. I have an unvented cathedral ceiling in the attic, with rafters filled with mineral wool. I would like to get a clearer picture of what is involved before describing my needs to prospective contractors.

我在气候5号地区，并且读过Holladay先生在寒冷天气下表现出不同类型的泡沫的帖子。此时，我倾向于EPS。或者可能是2英寸的Polyiso和3英寸的EP。但是我也听说过一些有关外墙的密集矿物羊毛板。屋顶有类似的产品吗？（如果是这样，您建议您进一步调查吗？）

Also, I don’t really have a clear picture of exactly how this is done. From what I’ve read, the foam is laid in overlapping layers, taped, and nailed to the rafters with long nails. Then new sheathing is laid on top -presumably on furring strips so that the top roof is vented like an umbrella over the foam. Is it really structurally sound for this entire assembly to be held in place with nails passing through foam?

我还没有在网上找到任何真正阐明此过程的东西 - 如果有人有这样的链接，这将是最感激的。Holladay的一篇文章中的随附照片给出了一个很好的主意 - 我当然计划向承包商展示。但是，它似乎在泡沫和护套的顶层之间没有任何空间。也许不是必需的？

Many thanks!
Hari

回答

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Replies

GBA Editor

Martin Holladay|Apr 10, 2014 10:50am|#1

Hari,
The best place for you to start is probably to watch this GBA video:
Exterior Insulation Retrofit — How to Install Foam On a Roof.

如果你有任何后续问题s after watching the video, feel free to post them here.
Kamboji|Apr 10, 2014 12:16pm|#2

Thanks a lot Martin- that 2 minute video was well worth the price of my free GBA Pro trial membership ;). Very helpful. The method in the video is not entirely consistent with the jpg image above. In the image one sees a wooden frame that holds the foam in place along the bottom edge.

I'd still be interested to hear whether mineral wool board might be a viable alternative on the roof. Roxul makes a product called Comfortboard IS which seems to be marketed for exterior walls; not sure about on the roof.

It still seems as if the whole roof assembly being held in place by screws through thick foam may be structurally unsound, though I have no experience with this kind of stuff.
GBA Editor

Martin Holladay|Apr 11, 2014 05:30am|#3

Hari,
问：“我仍然有兴趣听到矿物质羊毛板是否可能是屋顶上的可行替代品。”

A.矿物羊毛绝缘材料通常在屋顶护套的外侧使用，但这些装置通常位于平坦（低斜坡）屋顶上。有关典型安装，请参见附件图像。如果您有兴趣在陡峭的屋顶上使用这种方法，我会联系矿物质羊毛制造商以寻求建议。

Q. "It still seems as if the whole roof assembly being held in place by screws through thick foam may be structurally unsound."

A. You are mistaken. As I wrote in myFine Homebuildingarticle on the topic, "The type of fasteners used and the way they need to be spaced for the top layer of sheathing depend on the pitch of the roof and roof loads, particularly wind and snow loads. It’s fairly easy to find screws with a pullout-resistance rating exceeding 400 lb., even when they are secured just to plywood or OSB sheathing. The fastener rating increases if the screws are driven into the rafters. (Fastener sources include Wind-lock and FastenMaster, which manufactures HeadLok and OlyLog screws.)"

如果您对工程或结构细节有进一步的疑问，请咨询工程师。

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